During any five minutes of a walk down a city street a man has hundreds of visual images flashed upon the retina of the eye.  His eye sees every little line in the faces of the passers-by, every detail of their clothing, the decorations on the buildings, the street signs overhead, the articles in the shop-windows, the paving of the sidewalks, the curbings and tracks which he crosses, and scores of other objects to most of which the man himself is oblivious.  His ear hears every sound within hearing distance,—­the honk of every horn, the clang of every bell, the voices of the people and the shuffle of feet.  Some part of his mind feels the press of his foot on the pavement, the rubbing of his heel on his stocking, the touch of his clothing all over his body, and all those so-called kinesthetic sensations,—­sensations of motion and balance which keep him in equilibrium and on the move, to say nothing of the never-ending stream of messages from every cell of every muscle and tissue of his body.

Out of this constant rush of stimuli our man gives attention to only the smallest fraction.  Whatever is interesting to him, that he sees and hears and feels.  All other sensations he passes by as indifferent.  Unless they come with extraordinary intensity, they do not get over into his consciousness at all.

="Listening-in” on the Subconscious.= The subconscious mind knows and needs to know what is happening in the farthermost cell of the body.  It needs to know at any moment where the knees are, and the feet; otherwise the individual would fall in a heap whenever he forgot to watch his step.  It needs to know just how much light is entering the eye, and how much blood is in the stomach.  To this end it has a system of communication from every point in the body and this system is in constant operation.  Its messages never cease.  But these messages were never meant to be in the focus of attention.  They are meant only for the subconscious mind and are generally so low-toned as to be easily ignored unless one falls into the habit of listening for them.  Unless they are invested with a significance which does not belong to them, they will not emerge into consciousness as real sensations.

=Psychic Thresholds.= Boris Sidis has given us a word which has proved very useful in this connection.  The limit of sensitivity of a cell—­the degree of irritability—­he calls the stimulus-threshold.[59] As the wind must come in gusts to drive the rain in over a high doorsill, so must any stimulus—­an idea or a sensation—­come with sufficient force to get over the obstructions at the doorway of consciousness.  These psychic thresholds do not maintain a constant level.  They are raised or lowered at will by a hidden and automatic machinery, which is dependent entirely on the ideas already in consciousness, by the interest bestowed upon the newcomer.  The intensity of the stimuli cannot be controlled, but the interest we feel in them and the welcome given them are very largely a matter of choice.